I usually convert long pdfs to epub using Calibre. It's a free program and is easy to use. You could possibly put instructions for people that want to convert it either on the website, or at the beginning of the pdf.

Do you really want a book? I mean the beauty of HTML was that content is separate from formatting. Some will look at this on a desktop, some on a tablet & most these days on a phone. Is a book format the right choice now that you have to be on-line?